Simply the
best landscape generator available...and it's free for personal use. I
was one of the first people to own a registered copy when the option became
available just so I could make REALLY big images (and because I think
Matt Fairclough is doing a great job!) Grab the program at
www.planetside.co.uk

I just downloaded
the latest version of Terragen (thanks for the reminder, Kurt :-) and
had forgotten what fun it is to play with, especially if you add just
a wee bit of Photoshop filtering to the finished landscape! Top picture
has lots of tweaking, bottom one just has unsharp mask and levels.